Organic light emissive devices (OLEDs) generally comprise a cathode, an anode and an organic light emissive region between the cathode and the anode. Light emissive organic materials may comprise small molecular materials such as described in U.S. Pat. No. 4,539,507 or polymeric materials such as those described in PCT/WO90/13148. The cathode injects electrons into the light emissive region and the anode injects holes. The electrons and holes combine to generate photons at a recombination zone in the light-emissive region.
FIG. 1 shows a typical cross-sectional structure of an OLED. The OLED is typically fabricated on a glass or plastics substrate 1 coated with a transparent anode 2 such as an indium-tin-oxide (ITO) layer. The ITO coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and cathode material 4. Other layers may be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.
In one arrangement shown in FIG. 1, the substrate 1 and the anode 2 are transparent to allow light emitted by the electroluminescent organic layer 3 to pass therethrough. Such an arrangement is known as a bottom-emitting device. In another arrangement the cathode 4 is transparent so as to allow light emitted from the electroluminescent organic layer 3 to pass therethrough. Such an arrangement is known as a top-emitting device.
There has been a growing interest in the use of OLEDs in display applications because of their potential advantages over conventional displays. OLEDs have relatively low operating voltage and power consumption and can be easily processed to produce large area displays. On a practical level, there is a need to produce OLEDs which are bright and operate efficiently but which are also reliable to produce and stable in use.
The structure of the cathode in OLEDs is one aspect under consideration in this art. In the case of a monochrome OLED, the cathode may be selected for optimal performance with a single electroluminescent organic material. However, a full color OLED comprises red, green and blue light organic emissive materials. Such a device requires a cathode capable of injecting electrons into all three emissive materials, i.e. a “common electrode”.
Cathode 4 may be selected from materials that have a workfunction allowing injection of electrons into the electroluminescent layer. Other factors influence the selection of the cathode such as the possibility of adverse interactions between the cathode and the electroluminescent material. The cathode may consist of a single material such as a layer of aluminium. Alternatively, it may comprise a plurality of metals, for example a bilayer of calcium and aluminium as disclosed in WO 98/10621, elemental barium disclosed in WO 98/57381, Appl. Phys. Lett. 2002, 81(4), 634 and WO 02/84759 or a thin layer (1 to 15 nm) of dielectric material to assist electron injection, for example lithium fluoride disclosed in WO 00/48258 or barium fluoride, disclosed in Appl. Phys. Lett. 2001, 79(5), 2001. In order to provide efficient injection of electrons into the device, the cathode preferably has a workfunction of less than 3.5 eV, more preferably less than 3.2 eV, most preferably less than 3 eV.
A layer of metal fluoride located between the organic emissive layer (or organic electron transporting layer, if present) and the metal cathode can result in an improvement in device efficiency—see for example Appl. Phys. Lett. 70, 152, 1997. This improvement is believed to result from a reduction in the barrier height at the polymer/cathode interface, allowing improved electron injection into the organic layer(s). A mechanism of device degradation using the LiF/Al cathode is proposed in Appl. Phys. Lett. 79(5), 563-565, 2001 wherein LiF and Al may react to release Li atoms that can migrate into the electroluminescent layer and dope the electroluminescent material. However, the present inventors have found the LiF/Al cathode to be relatively stable, its main drawback being relatively low efficiency (in particular when used as a common cathode). A more efficient arrangement utilizes a tri-layer of LiF/Ca/Al, which is described as a common cathode in Synth. Metals 2000, 111-112, p. 125-128. However, it is reported in WO 03/019696 that degradation is particularly marked for devices comprising this cathode and fluorescent electroluminescent materials comprising sulfur such as the red emitting polymer comprising the trimer repeat unit thiophene-benzothiadiazole-thiophene. WO 03/019696 proposes using a barium based material rather than LiF and discloses a tri-layer structure of BaF2/Ca/Al for these fluorescent electroluminescent materials comprising sulfur. The use of other barium compounds including barium halides and barium oxide is also mentioned as a possibility in WO 03/019696. The barium compound layer is disclosed as having a thickness in the range 1 to 6 nm.
U.S. Pat. No. 6,563,262 proposes using a bilayer of a metal oxide (e.g. BaO) with aluminium for fluorescent poly(p-phenylene vinylene) emissive materials (PPVs). The metal oxide layer is disclosed as having a thickness in the range 1.5 to 20 nm.
In light of the above, it can be seen that there are various disclosures of using thin metal compound layers as electron-injecting layers in a cathode of an organic light emissive device. Thus, these layers do not provide good protection of the underlying layers when, for example, an overlying layer is deposited using a high-energy process such as sputtering.
WO 2006/016153 discloses the use of a composite electron-injecting layer comprising a metal compound and a metal. It is taught that such composite layers can reduce quenching by the metal component while retaining good electrical properties. It is further taught that these composite layers can be made with good transparency for top-emitting devices. It is further taught that the metal component increases the conductivity of the layer thus allowing thick, transparent, conductive layers to be provided which can act as a buffer layer (sputter barrier) for protecting underlying layers when a material such as ITO is sputtered thereover. However, a possible problem with these composite layers is that the co-deposition process used to form them is more expensive and difficult to control when compared with deposition of single components.
U.S. Pat. No. 6,576,093 discloses a bilayer cathode comprising a layer of a low workfunction material such as Ca and a layer of a higher workfunction material such as aluminium. It is described that a cathode layer is typically deposited by vacuum evaporation or by a sputtering technique such as rf sputtering or dc magnetron sputtering. It is described that when the underlying layer is a layer of a relatively sensitive material such as a soluble conjugated polymer, vacuum evaporation is often the preferred technique for depositing the first layer because it is a relatively low-energy process which causes less damage to the underlying layer of organic material. It is further described that cathode layers deposited by conventional vacuum evaporation techniques contain pinholes through which water and oxygen are able to enter the device and initiate reactions at the interface between the organic layer and the cathode. These reactions result in the formation of non-emitting black spots with a consequential degradation in device performance. Accordingly, it is suggested that the cathode should be formed by depositing a first layer of a low workfunction material using a low energy deposition technique such as vacuum evaporation and depositing a second layer of a higher workfunction material by a conformable deposition technique such as a sputtering technique.
An aim of embodiments of the present invention is to provide an alternative solution to the aforementioned problem of pinholes in the cathode layers leading to non-uniform light emission. A further aim is to provide an organic light emissive device structure which has increased opto-electrical efficiency. Yet a further aim is to provide an organic light emissive device structure which has a lower initial drive voltage and better drive voltage stability during storage and baking. A yet further aim is to provide an organic light emissive device with improved lifetime, in particular at elevated operating temperature.